The Earl of Orkney was originally a Norse jarl ruling Orkney, Shetland and parts of Caithness and Sutherland. The Earls were periodically subject to the kings of Norway for the Northern Isles, and later also to the kings of Alba for those parts of their territory in mainland Scotland (i.e. Caithness and Sutherland). The Earl's status as a Norwegian vassal was formalised in 1195. In 1232 a Scottish dynasty descended from the Mormaers of Angus replaced the previous family descended from the Mormaers of Atholl, although it remained formally subject to Norway.

The first known Earl of Orkney was Ragnvald Eysteinsson (Rognvald, Earl of Møre), who died around 890. Subsequent Earls, with one exception, were descended from Rognvald or his brother Sigurd until 1232.

The Norse Earldom was frequently under joint rule. The possessions of the Earldom included the Mormaerdom of Caithness and, until 1194, the Shetland Islands.